1. Field of the Invention
The present invention relates generally to desiccant tablets. More specifically, the present invention relates to an improved desiccant tablet using ethyl-vinyl acetate as a binder.
2. Description of Related Art
Any sorbent inserted into places such as disk drives, microelectronic devices, and the like, must maximize the amount of sorbent capacity within their bodies compared to their size because of limited size of the confines in which they are often placed. This demands a low profile type of sorbent, which excludes many traditional sorbent carrying devices such as canisters, capsules, and sachets. Compression of the sorbent, therefore, makes the most sense, since it will allow for the maximum amount of sorbent to be in a given space while keeping the space taken up by the sorbent low. Even with the benefit of a high absorbency per unit volume, compressing the sorbent brings its own group of factors that must be considered. First and foremost, the compressed sorbent must be clean. That means that it cannot be dusting or friable. Dusting occurs when particles that are loose or loosely bound to their neighbors are dislodged through minor abrasions or vibration, producing a find dust of particles. Friability, however, occurs when the compressed sorbent has particles dislodged through vigorous abrasion, either through vibration or agitation. Friability is determined as a percent loss based on the initial weight of the compressed sorbent minus the final weight of the compressed sorbent minus any absorbed moisture divided by the initial weight of the compressed sorbent and the result multiplied by 100. The actual test requires that the compressed sorbent is run through a Friability tester, such as a VanKel Friabilator. This type of tester has two rotating drums, one on either side of the unit that contain the test compressed sorbent, though any similarly operating friability tester would function as well. A standard test requires 200 revolutions.
Because of these two requirements, the sorbent must be tailored to hold as many of the particles together as possible, since most of the components a compressed sorbent would be placed in, such as electronics, cannot have small particles loose in their enclosed environments. Secondly the tablet must be substantially rigid, but still have some degree of flexibility. This allows for the part to retain its shape during transport, but not be so brittle that when it is fitted into the snug space that it is required to be placed, that the part shatters. Thirdly, there cannot be a high degree of variation between parts. Electronics manufacture requires a high degree of dimensional tolerance, due to the precise nature of construction of circuitry. These tolerance requirements even extend to housings that surround the electronics. Often, there is very little space available for the sorbent to be placed, as stated earlier. Since this space is so tight, and precise, the part must be able to have the same degree of tolerance, otherwise, the part could rattle around in the component and run the risk of having particles abrade off, or the part will just not fit, and be rejected. Thus, only when the part is deemed clean, fits snuggly, and has a low degree of dimensional variability, will the part be acceptable for use in a given device.
While the type of sorbent chosen, such as silica gel, molecular sieve, activated carbon, clay, or any combination thereof, does play a part in these aspects, the binder plays a key role. Typical binders used are polyethylene and polyvinylpyrrolidone (PVP). Polyethylene is a multifunctional polymer whose properties change with the degree of branching, degree of crystallinity and average molecular weight. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium-density and high-density polyethylene, the melting point is typically in the range 120-130° C. The melt point for average commercial low-density polyethylene is typically 105-115° C. Most Low Density Polyethylene, Medium Density Polyethylene, and High Density Polyethylene grades have excellent chemical resistance and do not dissolve at room temperature because of the crystallinity. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons (e.g. toluene, xylene) or chlorinated solvents (e.g. trichloroethane, trichlorobenzene). However, with the regular patterns of the chains, and a low possibility of substitution of side chains that would increase adhesion, polyethylene is not an ideal binder.
PVP is a white, hygroscopic powder. It has a high degree of solubility in both water and organic solvents, but is not so soluble in esters, ethers, hydrocarbons and ketones. PVP is notably quite adhesive to materials, which allow it to be used in film formation, and in specialty polymers. However, this high degree of solubility, while perfect for creating adhesive coatings or glues, does not make for an ideal solid binder, especially when ease of manufacturing is a concern.
For example, the current state of the art uses a PVP binder with activated carbon. While one can get a large degree of sorbent in the compressed body when rubbed, carbon particles come off onto the surface that it is rubbed against. More critically, the tolerance of the compressed sorbent does not meet the standard for the technology it is placed in. An activated carbon compressed sorbent made with PVP in the current state of the art has a tolerance of roughly 0.020 inches. The tolerance of a standard microelectronic device is 0.002 inches; a much higher degree of tolerance then currently available. To compensate for this, the current industry standard is to not attempt to maximize the amount of sorbent in the compressed body, as that maximization leads to having a large number of rejected parts that are too large to fit in the high dimensional tolerance spaces, because of the part's low dimensional tolerances. However, this is of course also undesirable, since having a lower amount of sorbent leads for a lower degree of absorption capability, and a lower amount of material as a whole lowers the dimensional size of the parts at the lower end of the size distribution, allowing them to be able to vibrate in the cavity, which leads to particulate creation. In an attempt to counteract this problem, multiple methods of manufacture have been utilized to hopefully enhance these requirements, but while still keeping with the traditional binder and sorbent combinations.
One such method requires that the carbon be mixed with a solution based PVP binder in a sigma mixer, then having the resulting mixture ground and classified due to agglomeration. After that, the mixture is compressed, and finally activated at 110 C. All of these steps are labor and energy intensive, and in the end, do not produce a highly clean part, that is dimensionally stable, and with a desirable degree of flexibility
Accordingly, there is a need in the art to create parts that are dust free, have a high degree of dimensional stability and can withstand abrasion forces that normally occur in their environments. There also is a need in the art for a more streamlined and time effective method of creating a formed sorbent body, resulting in a more efficient manufacturing process.